Part 5 — The FGF21 Playbook · Castore: Built to Adapt

Part 5 — The FGF21 Playbook

We’ve talked about what FGF21 is, when it rises, how it changes your mitochondria, and what science has shown us. Now let’s put it all together into something you can actually use. FGF21 isn’t a hormone you want blasting all the time. It works like a reset button short pulses of it help your mitochondria adapt and stay flexible. The key is to create the right contrast: some days that push it high, and other days that let it fall so your body stays sensitive.

Here are the main levers you can pull:

1. Strategic sugar or carb surges.

On training days, especially when you’re lifting hard or doing intervals, a surge of carbs with very low fat can spike FGF21. This is like revving your engine on purpose—it makes the mitochondria split into more units (fission) to keep up with demand.

2. Low-calorie or fasting days.

On recovery or rest days, pulling calories back especially carbs and protein helps mitochondria repair (fusion) and recycle damaged parts (mitophagy). This is like parking the car in the shop so it can get tuned up.

3. Exercise.

Whether it’s cardio, strength training, or even just a brisk walk, exercise is a universal trigger for FGF21. It’s like the master switch that makes sure your cells are ready to handle whatever fuel you throw at them.

4. Environmental stressors.

Cold showers, sauna, or even just walking outside in the winter can bump up FGF21. These little stressors teach your mitochondria to stay resilient.

5. Support stack.

Peptides like MOTS-c or SS-31, nutrients like CoQ10 and carnitine, and lifestyle choices like good sleep all improve how your body responds to FGF21. Think of these as the supporting cast that make the conductor’s job easier.

Sample Weekly Calendar (200-lb man, 4 training days)

Monday (Training – Fission Day)High carbs, low fat, lean protein. Example: fruit, juices, honey during the day; lean chicken and veggies at night.

Tuesday (Rest – Fusion Day)Low calorie, higher fat, lots of vegetables. Example: salad with salmon and olive oil, roasted broccoli, eggs.

Wednesday (Training – Fission Day)Similar to Monday—sugar surge around the workout, lean protein at dinner.

Thursday (Rest – Fusion Day)Time-restricted eating, fibrous veggies, omega-3 fats.

Friday (Training – Fission Day)Fruit-heavy, carb surge day with workout.

Saturday (Training – Fission Day)Repeat high-carb/low-fat fueling with post-workout carb surge.

Sunday (Rest – Fusion Day)Lighter intake overall, healthy fats, Zone 2 walk, early cutoff for eating.

Think about alternating between revving your car on the highway (fission days) and taking it into the shop for maintenance (fusion days). FGF21 is the system software that makes sure the engine knows how to do both. Without the rev, the engine gets sluggish. Without the tune-up, it burns out. The magic is in the cycle.

FGF21 is not a hormone to “boost” all the time. It’s a pulse signal. By cycling carb surges, fasting or recovery days, exercise, and small stressors, you teach your mitochondria to stay flexible, efficient, and resilient. That’s the real dance FGF21 wants to lead.

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